The present invention relates generally to spectrometers and spectrographs having optical elements in which the surfaces of the said elements include a common center of curvature. More specifically the invention pertains to a Dyson, xe2x80x9cunit magnification optical system without Seidel aberrationsxe2x80x9d, which includes concentric spectrometers and concentric spectrographs in which the Dyson optical system is applied.
Concentric optical systems such as described by J. Dyson, JSOA, xe2x80x9cUnit Magnification . . . Aberrationsxe2x80x9d, Vol. 49, No. 7, pp.713-716, provide large image fields free of Seidel aberrations and are thus able to form images of high quality and resolution. This optical arrangement has been applied to advantage by L. Mertz, Applied Optics, xe2x80x9cConcentric Spectrographsxe2x80x9d, Vol. 16, No. 12, pp. 3122-3124, and W. Slutter (EP 0 862 050 A2; 1998) to spectrometers and spectrographs to produce high quality spectral dispersion of optical energy.
Internal reflections of light on optical surfaces can degrade the quality of the image formed at the image aperture. The image can be degraded in a myriad of optical phenomenon as a result of internal reflections. On example of degradation is by the formation of multiple images of the object at the image aperture. Another example of degradation caused by internal reflections is the formation of interference fringes at the image aperture. Still another example in which the quality of the image may be degraded is in the loss of contrast or detection limit when reflected light from an optical surface impinges out of focus at the image aperture and does not contribute to the formation of the image of the object. The object in spectrometers is typically the entrance slit or entrance aperture through which the optical energy to be analyzed enters the spectrometer.
In a concentric spectrometer of the Dyson optical configuration, there are two specular surface reflections that contribute to, or give rise to, internal reflections. Both reflections originate at the convex surface of the plano-convex lens, which lies concentric to a concave diffraction grating. The first reflection occurs when light transmitted from the object plane falls incident on the convex surface of the lens and the second occurs when the diffracted light from the grating impinges on said convex surface. Either one or both of these reflections can degrade the image quality of the spectrometer.
The effects of internal reflections have been reduced in the prior art by the deposition of various antireflection coatings on the optical surfaces within the concentric spectrometer. An antireflective coating(s) may be applied to optical surfaces to reduce the differential change in refractive index when the ray propagates from one optical media such as air, to a second optical media of different refractive index such as glass, thus reducing the magnitude of the specular reflection. Many examples of coatings on optical surfaces exist that reduce the magnitude of reflections on optical surfaces. Indeed a great amount of literature has been devoted to the study of single, and multiple, layer depositions that reduce reflections on optical surfaces.
A high performance antireflection coating(s), usually a multi-layer dielectric coating that substantially reduces the reflection at a surface, has inherent disadvantages. The disadvantages include a high cost of production, a narrow range of wavelengths for which the reflection loss is low, limited angles of incidence in which rays may propagate with low reflection, and are fragile requiring special handling, cleaning and environmental considerations. Indeed, these high performance coatings can cause a greater magnitude of reflection than a surface without an antireflective coating when used beyond the wavelength range of design. Regardless of the coating(s) used, the internal reflections are not reduced over a wide wavelength range to a level which degradation of the image quality does not take place by one or more of the aforementioned optical phenomenon within concentric optical systems.
Another means in which internal reflections have been reduced in concentric spectrometers is by the reduction of the numerical aperture (NA) of the spectrometer. One way the NA of the spectrometer can be reduced is by the placement of an optical stop within the spectrometer. This is undesirable since the result is a loss in detectable signal, a decrease in the etendue or throughput of the optical system, and can add to or increase the stray light within the spectrophotometer since more energy must be absorbed within the confines of the spectrometer.
There is a need, therefore, for the mitigation of internally reflected light within a concentric spectrometer such that no degradation of the image quality or loss in contrast occurs yet high etendue is preserved.
The present invention overcomes the limitations of internal reflections in concentric spectrometer optics by mitigation of the specular reflected rays from propagating through the optical system. As a result, the quality of image at the detection or image plane is preserved while maintaining high etendue.
In accordance with the present invention an object and an image aperture are provided, through which optical radiation, or light, enters the spectrometer through said object aperture and a spatially dispersed image of said object aperture by wavelength is formed at said image aperture. The field or extent of the object and image apertures along with the focal ratio of the optical system defines the limit within which light may propagate the optical system. Rays that propagate the system at the limit of the aperture fields are the marginal rays.
A concave diffraction grating is provided that reflects and diffracts incident light from an object(s) from the object plane and participates in the formation of a spatially dispersed image of the object(s) by wavelength at the image plane. A plano-convex lens is provided through which light from the object plane is transmitted to the diffraction grating, where the light it is diffracted and reflected, then transmitted again through the same plano-convex lens to form a spectrally dispersed image of the object within the image plane. The convex surface of the lens and concave surface of the diffraction grating are concentric, or nearly so, about a common center of curvature. An optical axis is also provided that includes the center of curvature of the optical elements and extends through the mechanical axis of both the lens and diffraction grating. A radial distance is defined from the optical axis that includes both object and image apertures.
Meridional planes, which include the optical axis, are defined to be perpendicular to the x-y path of a given diffracted ray as that ray propagates from the grating towards the convex surface of the lens. A diffracted ray that falls incident on the convex lens surface of the lens will give rise to specular reflection. A ray that falls incident on the convex lens surface prior to intersection of the rays"" meridional plane will cause the specular reflection to be directed to impinge on the grating a second time. This ray can further propagate the optical system by means of zero order diffraction to form a spectra or part of a spectra within the image aperture which does not correlate to the spectra of interest. The result is an increase in stray light. A diffracted ray that intersects the rays"" meridional plane prior to impinging on the convex lens surface will cause the specular reflection to be directed away from the diffraction grating and image plane without possibility of secondary reflection or diffraction. A baffle or series of baffles and or surfaces containing light absorbing media can be used accordingly to prevent further propagation within the spectrometer without interference with the optical path of the spectrometer. The present invention excludes the possibility of the diffracted rays from impinging on the surface of the convex lens without prior intersection of the meridional plane, thus eliminating further propagation of internal specular reflections.
Another aspect of the invention is the use of baffles which need only prevent propagation of light from one direction since secondary diffraction or reflection does not occur on the grating surface. The baffles can therefore be optimized to more completely mitigate stray light by multiple impingement of the reflected light on the absorbing media or baffles to further reduce the total stray light within the spectrometer.
Further in accordance with the present invention, marginal rays extending from the object aperture toward the optical axis are not permitted to intersect the optical axis prior to the convex surface of the lens as internal reflections will result. Furthermore, marginal rays that extend away from the optical axis define the clear aperture of the optical system.
Another feature of the present invention is an aperture mask placed in close proximity to the concave surface of the grating. This aperture mask limits the NA of the spectrometer and additionally reduces stray light in the spectrometer. The edge or near periphery of the grating is typically not of optical quality. The edge of a grating is usually chamfered to decrease the frailty of the grating but can have the undesirable effect of scattering light towards image aperture. Also, the optical surface beyond the clear aperture of the grating includes defects from the replication or manufacturing processes. Thus an aperture mask provides a surface that is absorbent to light, serves to limit the NA of the spectrometer, and masks that region beyond the clear aperture of the grating to prevent the scattering of light towards the image aperture.
Further in accordance with the current invention, the object and image apertures lie in parallel or coincident plane(s) in close proximity to, or are included within, the planar surface of the plano-convex lens. The thickness of the plano-convex lens and lens radii, separation of the convex surface of the lens and concave surface of the diffraction grating, radii of the diffraction grating, and grating groove density are concurrently adjusted to minimize aberrations within the spectrometer apertures and provide the desired spatial dispersion. Furthermore, the radial distance from the optical axis at which the object and image apertures reside and the diameters of the lens, grating, and aperture mask are adjusted in accordance with the invention to provide the desired numerical aperture for the spectrometer while eliminating internal reflections.